I made converter that outputs native QML items and animations out of Spine exported json files. So in a way, this is not a runtime at all, but the end results are similar, but with an added benefit of having native QML code. This means you could make a skeleton animate inside another skeleton or make some parts of the animation be procedural. Or you could just like the native QML look.

Hi Nate, I have. I actually looked through irukandjis code quite a lot while building a ffd for the converter (which is not yet released). They accomplish a similar thing, but through different route and while irukandjis runtime is more straightforward (develoment-wise) it keeps the skeleton in Spine structure. The converter I created produces native QML items, which gives a lot of options to modify their inner workings. QML has path animations, can do procedural textures add item specific shaders to name a few.